Heretofore there have been known the following semi-automatic welding processes using a coated electrode (hereinafter referred to simply as a welding rod). However, they have both merits and demerits; in general, they are poor in welding ability and introduce bulkiness in welding machines. Hence none of the conventional processes are satisfactory in these points.
(1) Gravity Arc Welding
This welding process is one that performs welding while moving the welding rod along a horizontal weld line by lowering a holder of the welding rod along an inclined slide bar. This welding process is advantageous in that the manipulation-of-electrode ratio (=the length of weld bead formed in a unit time/the length of the welding rod molten in the unit time) is constant, but this process requires the lengthening of the slide bar corresponding to the length of the weld line, which makes the welding machine bulky and loses its portability, resulting in the process becoming poor in workability.
(2) Inclination Welding with Electrode Set of Lower Angle
This welding process is one that performs welding while moving the welding rod along the horizontal weld line by rotating the welding rod holder after setting the welding rod at an angle of 10.degree. to 12.degree. to the horizontal weld line. This welding process does not involve the use of the slide bar, and hence it can achieve the reduction of the size and weight of the welding machine, but as the mnipulation-of-electrode ratio varies in a moment and does not stay constant, the bead width does not become fixed. Since the initial setting angle of the welding rod is as small as 10.degree. to 12.degree., the arc length becomes large to cause a disorder of arc. This introduces the possibilities of spatter occurring and undercuts being formed, thus resulting in lowered welding ability of the process.
For the solution of the abovesaid defects of such prior art welding processes and welding machines therefor, the present inventions have previously proposed a gravity arc welding process in which the angle between the welding rod and the weld line is initially set small and changed after starting the welding and then held constant, and a welding machine for use in this process as set forth in Japanese Pat. Pub. No. 8864/76 gazette.
But this welding process and the welding machine therefor were developed for the application to horizontal fillet welding, so they are difficult of application to vertical down welding.
That is to say, according to this welding process, the welding rod holder is first rotated to a predetermined position and then lowered by gravity along an inclined guide rail and, in this while, the welding rod moves along a horizontal weld line to perform horizontal fillet welding semi-automatically. The manipulation-of-electrode ratio can be held constant or substantially constant. Since this welding process is intended primarily for the horizontal fillet welding, however, when applying this process to the vertical down welding, it is difficult to hold the guide rail in a vertical position and the welding rod goes down along the weld line, so that spatters violently scatter from that welding position over a wide range to aggravate the welding environment. Especially, the spatters contaminate the guide rail, stopping the sliding movement of the welding rod holder or bringing trouble on the sliding movement thereof.
Further, the welding machine previously proposed by the present inventors has been made small and lightweight by shortening the guide rail, but as the provision of the guide rail is a requisite, it is difficult to support the guide rail in the vertical position and since a support mechanism suitable for such a purpose has not been developed, this welding machine cannot be applied to the vertical down welding.
The present invention has for its object to settle the aforementioned defects and, specifically, it is to propose a vertical down welding process and a welding machine therefor which employ a support base which is able to firmly and surely support the guide rail at the lower end thereof during vertical down welding, small and light-weight, easily detachable and easy to carry.
Furthermore, the present invention relates to a vertical down welding machine which is provided with a very simple-structured positioning device that is capable of properly holding the relative positions of the welding rod and the vertical down weld line, ensuring excellent welding ability of the welding machine.